There Is No Perfect Ending
Remember when your heart was broken and you wanted her to tell you, just once, why she left you? Why she cheated on you? Why she didn't tell you this before, when there was still time? You said you wanted closure, but there's no such thing as a perfect closure. There's no perfect ending. That notion of a clean conclusion is a myth propagated by stories and movies.
They propagate this myth because they run on a schedule. They have to wrap things up by steering you toward a logical conclusion. So in that story's universe, everything happens for a reason, and every character behaves a particular way because they want something, either out of goodness or out of pure malice. But there's always a reason behind their behavior, which makes them easy to understand.
Reality, though, doesn't work on a schedule. There's no rush to wrap things up. In reality, a lot of things remain unresolved, unanswered, unclosed. Even if you get every answer, there will still be something you want to know. The questions never stop, because they're driven by emotion, and emotion takes time to resolve. That's why nothing you say to someone in the middle of a breakdown ever really lands: the breakdown itself is what's driving the need for answers.
There are no satisfying answers in this world, just as there are no clever arguments that can deliver ultimate satisfaction. Start treating life as a random string of events stitched together. Cause and effect exists, but it doesn't exist solely for you. More often than not, people make a mess, and you're the one left to clean it up, wondering why it couldn't have gone any other way.
The bitter pill to swallow is that life is random. It never moves in neat structures. Your idea of a perfect structure, a perfect ending, is rooted in your own need to feel better about yourself. If she's leaving you, you want that closure delivered on your terms, in a way that makes her feel like shit. If you're the one leaving her, you want no questions asked. You expect her to simply accept whatever nonsense you handed her.
The need for closure is a myth propagated by stories, because stories run on a schedule. You borrowed that myth because you wanted the upper hand in every situation. The harsh truth is that life is indifferent to what you believe or want. It's random, and sometimes you don't get a perfect ending. You have to accept it and move toward a better beginning.